The present invention concerns a method and a device for determining the depth of anesthesia in a patient to whom at least one anesthetic substance has been administered.
It is essential to measure the depth of anesthesia administered to a patient as precisely and objectively as possible for two reasons: first, to determine the optimal analgesic and amnesiac effects of the anesthetic agent and second, to minimize the risks of anesthesia.
Optimal conditions can be attained only if the depth of anesthesia can be precisely and objectively evaluated.
Several methods and devices presently exist which determine, to some extent, the depth of anesthesia in a patient. In most cases the anesthetist determines the depth of anesthesia empirically based on exterior symptoms of pain in the patient such as a sweaty forehead, excretion of tears, ocular myosis, or an extreme reaction of the cardiovascular system such as tachycardia or bradycardia, abnormal blood pressure, etc. However, since these parameters vary from patient to patient, evaluating the depth of anesthesia may depend heavily on the subjective judgment of the anesthetist.
One method of determining the depth of anesthesia is proposed in International Patent Publication No. WO 91/19453. This method involves repeatedly administering stimuli to the patient, recording the patient's electrical brain activity after each stimulus, processing the data electronically, and transforming the resulting data into indications of the depth of anesthesia. Since this method involves administering stimuli to the patient, it may not always be practical. Furthermore, such a response is independent of the patient's autonomous nervous system and does not take into account the instant state of the autonomous nervous system.
Another method and apparatus designed to furnish measurements of the depth of anesthesia in a patient are described in International Patent Publication No. WO 92/06632. In this method, a series of waves R is analyzed statistically to plot the modifications in cardiac frequency variations caused by respiration. The instant frequency is compared to an average frequency in order to quantify the depth of anesthesia.
In actuality, this method is generally useless if the patient is artificially ventilated during surgery. In this situation, physiological control of ventilation has no influence on the mechanics of respiration.